Spin-state ordering and magnetic structures in the cobaltites YBaCo2O5+δ (δ=0.50 and 0.44)

The antiferromagnetic-ferromagnetic phase transition in YBaCo2O5.50 and YBaCo2O5.44 cobaltites with different types of oxygen-ion ordering in the [YO0.5/0.44] layers has been studied by neutron powder diffraction in combination with group-theoretical analysis. As a result, the crystal and magnetic structures above and below the phase transformation temperature T-i were determined and successfully refined. In both cases, the proposed models involve a spin-state ordering between diamagnetic (t(2g)(6)e(g)(0), S=0) and paramagnetic (t(2g)(4)e(g)(2), S=2) Co3+ ions with octahedral coordination. Electronic ordering results in a nonzero spontaneous magnetic moment in the high-temperature magnetic phases with isotropic negative exchange interactions. In the case of YBaCo2O5.5, the phase transformation does not change the Pmma (2a(p) x 2a(p) x 2a(p)) symmetry of the crystal structure. The wave vectors of magnetic structures above and below T-i are k=0 and k=c(*)/2, respectively. In the case of YBaCo2O5.44 a crossover P4/nmm (3 root 2a(p) x 3 root 2a(p) x 2ap) -> I4/mmm (3 root 2a(p) x 3 root 2a(p) x 4a(p)) was involved to solve the low-temperature magnetic structure. The wave vectors in both high-temperature and low-temperature magnetic phases are k=0. Mechanisms of the phase transformation in both compositions are discussed in the light of obtained magnetic structures. The proposed spin configurations were compared with other models reported in literature.